The glasses of the present invention are designed for making seals to vitreous materials of high alkali metal content such as are required for certain electrochemical, electrical, electronic, and optical applications. One example of an electrochemical application is the use of sodium ion-conducting glass membranes or electrolytes in one form of sodium-sulfur, high efficiency battery. Such glasses contain a high content of Na.sub.2 O and require low temperature, non-distorting seals. In the electrical, electronic, and optical areas, there are similar requirements for sealing to flat soda lime glass substrates utilized for circuit boards or as transparent enclosures for display systems, microcircuits, magnetic or optical storage matrices, solar energy conversion devices, etc.
An application of particular interest for the present inventive glasses is for sealing to the sodium ion-conducting glasses employed as solid electrolytes in sodium-sulfur batteries. In brief, such a battery comprises a liquid anode, a liquid or paste-like cathode, and a solid electrolyte-separator or membrane which is selectively permeable and which separates the anode and cathode. The anode consists of a molten alkali metal, an alkali metal amalgam, or a solution. The cathode consists of liquid sulfur and sulfides of the anode metal. The membrane is selectively permeable to the cations of the anode metal without transmitting, to any appreciable extent, other ions, neutral molecules, or electrons from either the anode or cathode. The anode, cathode, and membrane elements are customarily positioned within a liquid and vapor tight case.
Commonly, liquid sodium has comprised the anode, a liquid sulfur-sodium sulfide mixture, e.g., a sodium polysulfide, has comprised the cathode, and the membrane has been formed from a sodium ion conductive ceramic, such as .beta.-alumina, or a glass. The use of sodium sulfide dissolved in, or in admixture with, liquid sulfur as the cathode has been found to be particularly useful since those two components form a wide range of mixtures that become liquid at relatively low temperatures, i.e., between about 235.degree.-285.degree. C., and demonstrate high electrical conductivity over a certain range of composition.
Sodium and sulfur are readily available and relatively inexpensive, when compared with lead. Furthermore, batteries made therefrom yield several times more energy per unit weight than the active materials of lead and lead oxide in the conventional lead-acid battery. Accordingly, the sodium-sulfur battery has the potential to replace the lead-acid battery in many applications.
As described in U.S. Pat. No. 3,679,480, one construction of an alkali metal-sulfur battery contemplates an electrolyte-separator (membrane) in the form of a bundle of very thin, hollow glass fibers (capillaries) sealed with open end into a common header. The capillaries are closed at the other end which is immersed into the molten alkali metal sulfide contained in the cathode chamber and are filled with the liquid alkali metal and in open communication with the anode compartment. The capillaries are prepared from a sodium ion-conducting glass. The sealing glass completes the separation between the anodic and cathodic liquids while mechanically holding the capillaries together in a bundle.
In order to obtain a good seal in that application, the sealing glass must fulfill the following three general requirements:
(1) the softening point of the sealing glass must be considerably below the strain point of the capillary glass to obtain a leak tight seal without thermal deformation of the capillaries during the sealing process;
(2) the coefficients of thermal expansion of the sealing glass and the capillaries should be well matched to insure a relatively stress-free seal; and
(3) the seal must be strongly resistant to attack by liquid alkali metal and alkali metal sulfide during battery operation.
A further property most desirable in the sealing glass is an annealing point slightly above the operating temperature of the battery (.about.300.degree. C.) to insure seal stability against deformation caused by internal pressure differences occurring during battery operation.
In conventional glass sealing, the sealing glass has to be heated to a temperature at which it is sufficiently fluid to wet the glass to which it is to be sealed in order to form a strong bond to it. Either one of two general mechanisms is involved in forming a bond: (1) simple surface adhesion; or (2) the formation of a common boundary layer through a diffusion process. In order to achieve the fluidity necessary for good wetting and for a tight bond, the sealing glass must be heated well above the softening point thereof, i.e., to a temperature at which the glass manifests a viscosity less than 10.sup.7 poises and, preferably, as low as 10.sup.2 -10.sup.4 poises. This practice is beset with several disadvantages, for example:
(a) the fluid sealing glass flows out of place or deforms unless it is contained and containment is not feasible in numerous applications;
(b) the glass to which the sealing glass is bonded must have a higher annealing temperature or strain point than the sealing temperature, i.e., the temperature at which the sealing glass has to be quite fluid, this factor making it necessary to have a large difference in viscosities existing between the two glasses which circumstance, in turn, eliminates many potentially useful combinations of glasses having but small differences in viscosities; and
(c) the depth of the boundary layer between the two glasses is extremely difficult to control.
In general, a boundary layer will be formed which must be very thin and uniform in depth so as to minimize stresses which may be developed therein if the thermal expansion of the boundary layer does not closely match the thermal expansions of the two glasses being sealed together.
Table I reports the composition and several physical properties for a glass which has been employed as the membrane for a sodium-sulfur storage battery and a sealing glass that has been used therewith. The glass compositions are tabulated in terms of weight percent on the oxide basis as calculated from the batch. The coefficient of thermal expansion (Expansion) is reported for the range of room temperature (R.T.), commonly about 25.degree. C., to 250.degree. C.
TABLE I ______________________________________ Membrane Sealing Glass ______________________________________ Na.sub.2 O 27.8 4.5 B.sub.2 O.sub.3 62.6 92.7 SiO.sub.2 5.4 2.8 NaCl 4.2 -- Expansion (.times. 10.sup.-7 /.degree.C.) 124 126 Softening Point (.degree.C.) 532 396 Annealing Point (.degree.C.) 456 280 Strain Point (.degree.C.) 429 248 ______________________________________
Those two glasses satisfy the three general requirements listed above for such glasses. It has been observed, nevertheless, that battery failure occurs after a relatively short period of operation and is usually the result of breakage of the membrane-capillaries at the interface with the sealing glass. Customarily, a sealing temperature of about 380.degree. C. was utilized with that set of glasses but that temperature produced seals of only marginal quality, particularly in the case of larger (&gt;1") seal areas, with regard to leak tightness. However, it was noted that, where sealing temperatures in the neighborhood of 390.degree.-400.degree. C. were employed to improve leak tightness, many capillaries exhibited high brittleness at the seal. These observations led to the conclusion that either prolonged use at battery operating temperatures (.about.300.degree. C.) or excessive sealing temperatures cause some unexplained interaction between the sealing glass and the membrane glass resulting in mechanical weakness and eventual failure.